It is known to position a semi-conductor such as tin oxide in a gaseous stream to detect combustibles such as carbon monoxide and/or hydrocarbons. An assumed operating mode of an "oxidizing" type semi-conductor sensor involves the adsorption of oxygen on the porous sensor surface to determine the sensor's electrical conductivity by affecting the population of electrons in the semi-conductor's conduction band; the oxygen weakly holds electrons which would otherwise be available in the conduction population. When the oxygen is surrendered to combustible gases it frees electrons for conduction, reducing the resistance of the sensor and increasing current flow therethrough.
For best operation and reasonable service life the semi-conductor materials known to applicant should be maintained in a temperature range between about 200.degree.F and 300.degree.F. Unfortunately engine exhaust temperatures are usually higher than this range. Therefore the exhaust gases should be cooled before passage across the semi-conductor bead. Cooling of the gases is also required or beneficial when using other types of sensors, such as electrochemical sensors (electrolysis of copper from copper borofluoxide electrolyte), infrared sensors, or spectrometric sensors.
The present invention deals with a conduit arrangement for cooling an engine exhaust gas sample before its passage over a combustibles sensor. The arrangement contemplates mechanism for diverting a minor fraction of the hot gas from the exhaust duct into a mixing chamber that communicates with a source of cooling air (such as the ambient atmospheric). The gas-air mixture is passed over the sensor to provide the readout.
Motive force for moving the diverted gas and coolant is provided by a restriction in the engine exhaust duct. Such restriction forms a superatmospheric pressure zone at an upstream location, and a subatmospheric pressure zone at a downstream location. The aforementioned mixing chamber communicates with the two zones so that the pressure differential (between the two zones) produces gas flow through the chamber. Coolant air flow is produced by the inductive effect of the subatmospheric pressure zone on the ambient. The arrangement uses no moving parts, with the possible exception of certain control valves that might prove necessary or desirable in some instances.